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Publications + Abstracts (2003-2008)

Target-free Stereo PIV : a novel technique with inherent error estimation and improved accuracy

by A. Fouras, D. Lo Jacono and K. Hourigan
in Experiments in Fluids, 44(2), 317-329 (2008). doi link

Abstract : A novel, accurate and simple stereo particle image velocimetry (SPIV) technique utilising three cam- eras is presented. The key feature of the new technique is that there is no need of a separate calibration phase. The calibration data are measured concurrently with the PIV data by a third paraxial camera. This has the benefit of improving ease of use and reducing the time taken to obtain data. This third camera also provides useful velocity information, considerably improving the accuracy of the resolved 3D vectors. The additional redundancy provided by this third perspective in the stereo reconstruction equations suggests a least-squares approach to their solution. The least-squares process further improves the utility of the technique by means of the reconstruction residual. Detailed error analysis shows that this residual is an accurate predictor of resolved vector errors. The new technique is rigorously validated using both pure translation and rotation test cases. However, while this kind of validation is standard, it is shown that such validation is substantially flawed. The case of the well-known confined vortex breakdown flow is offered as an alternative validation. This flow is readily evaluated using CFD methods, allowing a detailed comparison of the data and evaluation of PIV errors in their entirety for this technique.

Measurement of instantaneous velocity and surface topography in the wake of a cylinder at low Reynolds number

by A. Fouras, D. Lo Jacono, G.J. Sheard and K. Hourigan
in Journal of Fluids and Structures, 24(8), 1271-1277 (2008). doi link

Abstract : A technique capable of simultaneous measurement of free-surface topography and velocity vector field data is presented. This technique offers substantial benefits of both reduced complexity and enhanced accuracy over all other techniques known to offer the same measurements. The flow behind a circular cylinder at low Reynolds numbers is measured using this technique. The velocity and vorticity fields as well as Strouhal number closely match the expected results. The free-surface topography, which can be related to the pressure field, exhibits an intimate relationship to the vorticity field.

Control of vortex breakdown in a closed cylinder with a small rotating rod

by D. Lo Jacono, J.N. Sørensen, M.C. Thompson and K. Hourigan
in Journal of Fluids and Structures, 24(8), 1278-1283 (2008). doi link

Abstract : Effective control of vortex breakdown in a cylinder with a rotating lid was achieved with small rotating rods positioned on the stationary lid. After validation with accurate measurements using a novel stereoscopic particle image velocimetry (SPIV) technique, analysis of numerical simulations using a high-order spectral element method has been undertaken. The effect of a finite length rod creates additional source terms of vorticity as the rod rotates. These additional source terms and their spatial locations influence the occurrence of the vortex breakdown.

Scaling of cell size in cellular instabilities of nonpremixed jet flames

by D. Lo Jacono and P.A. Monkewitz
in Combustion and Flame, 151, 321-332 (2007). doi link

Abstract : Systematic experiments have been undertaken to study the parameter dependence of cellular instability and in particular the scaling of the resulting cell size in CO2-diluted H2–O2 jet diffusion flames. Cellular flames are known to arise near the extinction limit when reactant Lewis numbers are relatively low. The Lewis numbers of the investigated near-extinction mixtures, based on the initial mixture strength φm and ambient conditions, varied in the ranges [1.1–1.3] for oxygen and [0.25–0.29] for hydrogen (φm is defined here as the fuel-to-oxygen mass ratio, normalized by the stoichiometric ratio). The experiments were carried out both in an axisymmetric jet (AJ) burner and in a two-dimensional slot burner known as a Wolfhard–Parker (WP) burner with an oxidizer co-flow (mostly 100% O2) of fixed low velocity. First, the region of cellular flames adjacent to the extinction limit was characterized in terms of initial H2 concentration and fuel jet velocity, with all other parameters fixed. Then, the wavelength of the cellular instability, i.e., the cell size, was determined as a function of the fuel jet velocity and the initial mixture strength φm. For conditions not too close to extinction, this wavelength is found to increase with the square root of the vorticity thickness of the jet shear layer and roughly the 1/5 power of φm. Very close to extinction, this scaling breaks down and will likely switch to a scaling with the flame thickness, i.e., involving the Damköhler number.

An experimental realization of an unstrained, planar diffusion flame

by D. Lo Jacono, P. Papas, M. Matalon and P.A. Monkewitz
in Proceedings of the Combustion Institute, 30, 501-509 (2005). doi link

Abstract : Abstract
A unique burner was constructed to experimentally realize a one-dimensional unstrained planar non- premixed flame, previously considered only in idealized theoretical models. One reactant, the fuel mixture in the current experiments, is supplied through a porous plug at the bottom of the combustion chamber and flows vertically up towards the horizontal flame. The crux of the design is the introduction of the oxidizer from above in such a way that its diffusion against the upward product flow is essentially one-dimensional, i.e., uniform over the burner cross-section. This feature was implemented by introducing the oxidizer into the burner chamber from the top through an array of 625 closely spaced hypodermic needles, and allowing the hot products to escape vertically up through the space between the needles. Due to the injection of oxidizer through discrete tubes, a three-dimensional ‘‘injection layer’’ exists below the exit plane of the oxidizer supply tubes. Experimental evidence suggests that this layer is thin and that oxidizer is supplied to the flame by 1-D counterdiffusion, producing a nearly unstrained flame. To characterize the burner, flame position measurements were conducted for different compositions and flowrates of H2–CO2 and O2–CO2 mixtures. The measured flame locations are compared to an idealized one-dimensional model in which only diffusion of oxidizer against the product flow is considered. The potential of the new burner is demonstrated by a study of cellular structures forming near the extinction limit. Consistent with previous investigations, cellular instabilities are shown to become more prevalent as the initial mixture strength and/ or the Damköhler number are decreased. As the extinction limit is approached, the number of cells was observed to decrease progressively.

Weak-inertial flow between two rough surfaces

by D. Lo Jacono, F. Plouraboué and A. Bergeon
in Physics of Fluids, 17(6), 063602 (2005). doi link

Abstract : “Oseen-Poiseuille” equations are developed from an asymptotic formulation of the three-dimensional Navier-Stokes equations in order to study the influence of weak inertia on flows between rough surfaces. The impact of the first correction on macroscopic flow due to inertia has been determined by solving these equations numerically. From the numerical convergence of the asymptotic expansion to the three-dimensional Navier-Stokes flows, it is shown that, at the macroscopic scale, the quadratic correction to the Reynolds equation in the weak-inertial regime vanishes generalizing a similar result in porous media.

Cell formation in non-premixed, axisymmetric jet flames near extinction

by D. Lo Jacono, P. Papas and P.A. Monkewitz
in Combustion Theory and Modelling, 7(4), 635-644 (2003). doi link

Abstract : Systematic experiments with CO2 diluted H2–O2 circular jet diffusion flames have been undertaken to study the formation of cellular flames, which occur for relatively low reactant Lewis numbers and near the extinction limit. The jet Reynolds number for all experiments was about 500, based on the centreline velocity, jet diameter and ambient fuel properties. The Lewis numbers, based on the initial mixture strength φ and ambient conditions of the investigated near-extinction mixtures, vary in the range 1.1–1.3 for oxygen and 0.25–0.29 for hydrogen (φ is defined here as the fuel-to-oxygen molar ratio normalized by the stoichiometric value). Various conditions near the extinction limit were investigated by fixing the fuel composition (H2–CO2 mixture), and systematically lowering the oxygen concentration in the co-flowing oxidizer stream past the point where cellular structures formed, until extinction occurred. The observed different instability states were correlated with the initial mixture strength and the proximity to the extinction limit.
The parameter space for cellularity was found to increase with decreasing initial mixture strength. For a given initial mixture strength, several cellular states were found to co-exist near the extinction limit, and the preferred number of cells (the azimuthal wave number) was observed to decrease with decreasing oxygen concentration (Damköhler number). These trends are consistent with previous theoretical work and our own stability analysis that will be reported elsewhere.